The invention relates to a soil-repellent monofilament for paper machine wire-cloths, having a diameter from 0.2 to 0.7 mm, of at least 85% by weight of polyethylene terephthalate having a viscosity index (V.I.) of at least 96 dl/g and a copolymer, processes for production, and its use.
By "soil-repellent" properties for paper machine wire-cloths are meant the wet soiling, which also predicates hydrolysis resistance. The soil repellency is intended to lengthen the cleaning cycles of a wire-cloth, the cleaning work being done under a high-pressure water jet and if necessary under friction with a brush. Soil-repellent monofilaments which should also be hydrolysis-resistant at the same time are woven into industrial cloths and used in particular in the dryer part of a paper machine. Such drying wire-cloths are exposed to temperatures far above 100.degree. C. and moisture and, during cleaning, also to increased mechanical stress. Soil-repellent polyester monofils have failed because of their ease of hydrolysis. The addition of additives in the form of stabilizers brought about a further improvement in the hydrolysis resistance, but it is still not sufficient for paper machine wire-cloths of the dryer part. Apart from adding additives, other attempts to enhance the soil-repellent properties include using suitable spin finishes, finishing treatments on the filament, and aftertreatment on the cloth.
Similarly, monofilaments were produced from the copolymer ethylenetetrafluoroethylene, whose thermal and chemical resistance are excellent and are therefore used in chemical process technology. Paper machine dryer wire-cloths are known which carry such monofilaments at the cloth surface and which protect the polyester monofilament base fabric underneath from soiling. However, the excellent chemical properties of the fluoropolymers are coupled with their inadequate tensile strength and elongation properties. Moreover, the production and disposal of a fluoropolymer having a fluorine content of about 50% is very costly compared with a polyester. Attempts have already been made to combine the mechanical properties of a mechanically stable a filament with the desirable chemical properties of a fluoropolymer by sheathing the mechanically stable filament by coextrusion with a fluorine-containing polymer (De-A-3301270). The sheath was kept as thin as possible for economic reasons. However, the known core-sheath threads are not suitable for use as paper machine wire-cloths with repeated use and cleaning under the rough conditions in the operation of a paper mill.